Method, system and device for integration of volatile and non-volatile memory bitcells

ABSTRACT

Disclosed are methods, systems and devices for operation of memory device. In one aspect, volatile memory bitcells and non-volatile memory bitcells may be integrated to facilitate transfer of stored values between the volatile and non-volatile memory bitcells.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. (TBD, Attorney Docket No. 252.P079), titled “METHOD, SYSTEM AND DEVICE FOR INTEGRATION OF VOLATILE AND NON-VOLATILE MEMORY BITCELLS,” filed on Apr. 23, 2018, and incorporated herein by reference in its entirety.

BACKGROUND 1. Field

Disclosed are techniques for utilizing memory devices.

2. Information

Non-volatile memories are a class of memory in which the memory cell or element does not lose its state after power supplied to the device is removed. The earliest computer memories, made with rings of ferrite that could be magnetized in two directions, were non-volatile, for example. As semiconductor technology evolved into higher levels of miniaturization, the ferrite devices were abandoned for the more commonly known volatile memories, such as DRAMs (Dynamic Random Access Memories) and SRAMs (Static-RAMs).

One type of non-volatile memory, electrically erasable programmable read-only memory (EEPROM) devices have large cell areas and may require a large voltage (e.g., from 12.0 to 21.0 volts) on a transistor gate to write or erase. Also, an erase or write time is typically of the order of tens of microseconds. One limiting factor with EEPROMs is the limited number of erase/write cycles to no more than slightly over 600,000—or of the order of 10⁵-10⁶. The semiconductor industry has eliminated a need of a pass-gate switch transistor between EEPROMs and non-volatile transistors by sectorizing a memory array in such a way that “pages” (e.g., sub-arrays) may be erased at a time in EEPROMs called flash memory devices. In flash memory devices, an ability to keep random access (erase/write single bits) was sacrificed for speed and higher bit density.

More recently, FeRAMs (Ferroelectric RAMs) have provided low power, relatively high write/read speed, and endurance for read/write cycles exceeding 10 billion times. Similarly, magnetic memories (MRAMs) have provided high write/read speed and endurance, but with a high cost premium and higher power consumption. Neither of these technologies reaches the density of flash memory devices, for example. As such, flash remains a non-volatile memory of choice. Nevertheless, it is generally recognized that flash memory technology may not scale easily below 65 nanometers (nm); thus, new non-volatile memory devices capable of being scaled to smaller sizes are actively being sought.

Technologies considered for the replacement of flash memory devices have included memories based on certain materials that exhibit a resistance change associated with a change of phase of the material (determined, at least in part, by a long range ordering of atoms in the crystalline structure). In one type of variable resistance memory called a phase change memory (PCM/PCRAM) devices, a change in resistance occurs as the memory element is melted briefly and then cooled to either a conductive crystalline state or a non-conductive amorphous state. Typical materials vary and may include GeSbTe, where Sb and Te can be exchanged with other elements of the same or similar properties on the Periodic Table. However, these resistance-based memories have not proved to be commercially useful because their transition between the conductive and the insulating state depends on a physical structure phenomenon (e.g., melting at up to 600 degrees C.) and returning to a solid state that cannot be sufficiently controlled for a useful memory in many applications.

Another variable resistance memory category includes materials that respond to an initial high “forming” voltage and current to activate a variable resistance function. These materials may include, for example, Pr_(x)Ca_(y)Mn_(z)O_(ϵ), with x, y, z and e of varying stoichiometry; transition metal oxides, such as CuO, CoO, VON, NiO, TiO₂, Ta₂O₅; and some perovskites, such as Cr; SrTiO₃. Several of these memory types exist and fall into the resistive RAMs (ReRAMs) or conductive bridge RAMS (CBRAM) classification, to distinguish them from the chalcogenide type memories. It is postulated that resistance switching in these RAMs is due, at least in part, to the formation of narrow conducting paths or filaments connecting the top and bottom conductive terminals by the electroforming process, though the presence of such conducting filaments is still a matter of controversy. Since operation of a ReRAM/CBRAM may be strongly temperature dependent, a resistive switching mechanism in a ReRAM/CBRAM may also be highly temperature dependent. Additionally, these systems may operate stochastically as the formation and movement of the filament is stochastic. Other types of ReRAM/CBRAM may also exhibit unstable qualities. Further, resistance switching in ReRAM/CBRAMs tends to fatigue over many memory cycles. That is, after a memory state is changed many times, a difference in resistance between a conducting state and an insulative state may change significantly. In a commercial memory device, such a change may take the memory out of specification and make it unusable.

SUMMARY

Briefly, one particular implementation is directed to bitcell circuit comprising: one or more volatile memory elements; one or more non-volatile memory elements, wherein a first volatile memory element of the one of the one or more volatile memory elements and at least a first non-volatile memory element of the one of the one or more non-volatile memory elements are accessible via a first bitline responsive to a signal on a wordline.

Another particular implementation is directed to a method comprising: accessing a first non-volatile memory element in a bitcell circuit, the bitcell circuit comprising at least the first non-volatile memory element and a first volatile memory element accessible via a first bitline; and transferring a first value stored in the first non-volatile memory element to the first volatile memory element while the bitcell circuit is disconnected from the first bitline.

Yet another particular implementation is directed to a memory circuit comprising: one or more volatile memory elements enabling the bitcell circuit to operate in a volatile bitcell mode, the one or more volatile memory elements being accessible by at least a first bitline; and a circuit to selectively couple one or more non-volatile memory elements between the at least a first bitline and a voltage source to transition operation of the bitcell circuit from the volatile bitcell mode to a non-volatile bitcell mode.

It should be understood that the aforementioned implementations are merely example implementations, and that claimed subject matter is not necessarily limited to any particular aspect of these example implementations.

BRIEF DESCRIPTION OF THE DRAWINGS

Claimed subject matter is particularly pointed out and distinctly claimed in the concluding portion of the specification. However, both as to organization and/or method of operation, together with objects, features, and/or advantages thereof, it may best be understood by reference to the following detailed description if read with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a computing device according to an embodiment;

FIGS. 2, 3A and 3B are schematic diagrams of memory systems comprising volatile memory bitcells and non-volatile memory bitcells according to an embodiment;

FIG. 4 is a diagram illustrating timing of operations to copy states between volatile memory bitcells and non-volatile memory bitcells according to an embodiment;

FIGS. 5 and 6 are schematic diagrams of memory systems integrating volatile memory bitcells and non-volatile memory bitcells according to an embodiment on common wordlines according to an embodiment;

FIGS. 7A and 7B are diagrams illustrating timing of operations to copy states between volatile memory bitcells and non-volatile memory bitcells according to an embodiment;

FIG. 8 is a schematic diagram of a memory system integrating volatile memory bitcells and non-volatile memory bitcells according to an embodiment;

FIG. 9 is a schematic diagram illustrating an addressing scheme integrating volatile memory bitcells and non-volatile memory bitcells according to an embodiment;

FIG. 10 is a schematic diagram integrating a smaller array of non-volatile memory bitcells with a larger array of volatile memory bitcells according to an embodiment;

FIG. 11 is a schematic diagram integrating a smaller array of volatile memory bitcells with a larger array of non-volatile memory bitcells according to an embodiment;

FIG. 12 is a schematic diagram illustrating an interleaving of volatile memory bitcells and non-volatile memory bitcells according to an embodiment;

FIGS. 13A through 13E are schematic diagrams of a bitcell circuit comprising volatile memory elements and non-volatile memory elements according to an embodiment;

FIG. 14A shows a plot of current density versus voltage for a CES device according to an embodiment;

FIG. 14B is a schematic diagram of an equivalent circuit to a CES device according to an embodiment; and

FIG. 15 is a schematic diagram of a three-dimensional integrated circuit structure integrating volatile memory bitcells and non-volatile memory bitcells according to an embodiment.

Reference is made in the following detailed description to accompanying drawings, which form a part hereof, wherein like numerals may designate like parts throughout that are identical, similar and/or analogous. It will be appreciated that the figures have not necessarily been drawn to scale, such as for simplicity and/or clarity of illustration. For example, dimensions of some aspects may be exaggerated relative to others. Further, it is to be understood that other embodiments may be utilized. Furthermore, structural and/or other changes may be made without departing from claimed subject matter. References throughout this specification to “claimed subject matter” refer to subject matter intended to be covered by one or more claims, or any portion thereof, and are not necessarily intended to refer to a complete claim set, to a particular combination of claim sets (e.g., method claims, apparatus claims, etc.), or to a particular claim. It should also be noted that directions and/or references, for example, such as up, down, top, bottom, and so on, may be used to facilitate discussion of drawings and are not intended to restrict application of claimed subject matter. Therefore, the following detailed description is not to be taken to limit claimed subject matter and/or equivalents.

DETAILED DESCRIPTION

References throughout this specification to one implementation, an implementation, one embodiment, an embodiment, and/or the like means that a particular feature, structure, characteristic, and/or the like described in relation to a particular implementation and/or embodiment is included in at least one implementation and/or embodiment of claimed subject matter. Thus, appearances of such phrases, for example, in various places throughout this specification are not necessarily intended to refer to the same implementation and/or embodiment or to any one particular implementation and/or embodiment. Furthermore, it is to be understood that particular features, structures, characteristics, and/or the like described are capable of being combined in various ways in one or more implementations and/or embodiments and, therefore, are within intended claim scope. In general, of course, as has been the case for the specification of a patent application, these and other issues have a potential to vary in a particular context of usage. In other words, throughout the disclosure, particular context of description and/or usage provides helpful guidance regarding reasonable inferences to be drawn; however, likewise, “in this context” in general without further qualification refers to the context of the present disclosure.

According to an embodiment, a computing device or computing platform may incorporate volatile memory systems and non-volatile memory systems to perform computing operations. In one embodiment, a memory device may comprise a “volatile” memory device that may maintain a particular memory state while power is applied to the volatile memory device, but may lose the particular memory state if power is removed. In another embodiment, a memory device may comprise a “non-volatile” memory that may maintain a particular memory state even after power is removed from the memory device.

A volatile memory system or non-volatile memory system may maintain memory states to represent values, symbols, parameters and/or conditions as memory states such as “bitcells.” In this context, a “bitcell” or “bitcell circuit” as referred to herein comprises a circuit or portion of a circuit capable of representing a value, symbol or parameter as a state. For example, a bitcell may comprise one or more memory devices that are capable of representing a value, symbol or parameter as a memory state of the memory device. In particular implementations, a volatile memory device may be made up of “volatile memory” bitcells that may lose a detectable memory state after power is removed from the volatile memory bitcells. Likewise, a non-volatile memory device may be made up of “non-volatile memory” bitcells capable of maintaining a detectable memory state after power is removed from the non-volatile memory bitcells.

According to an embodiment, a computing device or computing platform may include both non-volatile memory devices and volatile memory devices. In particular implementations, such a computing device or computing platform may copy or transfer memory states or stored values read from a volatile memory device to a non-volatile memory. Likewise, such a computing platform or computing device may copy memory states read from a non-volatile memory to a volatile memory device. Copying memory states or transferring stored values between volatile and non-volatile memory devices may entail latencies and power consumption affecting performance of a computing platform or device. Additionally, copying memory states or transferring stored values between volatile and non-volatile memory devices may impact memory bus resources used to transfer values between physical devices. Particular implementations described herein are directed to a coupling of volatile memory bitcells and non-volatile memory bitcells to reduce power consumption and latency in connection with copying memory states between volatile and non-volatile memory devices.

FIG. 1 is a schematic diagram of a computing device 100 according to an embodiment. A processor/controller 104 may execute processes or procedures (e.g., under control of computer-readable instructions) to perform various tasks including, for example, storing values in or reading values from addressable portions of memory 108. In a particular implementation, processor/controller 104 may communicate with a memory controller 106 through bus 102 according to a predefined interface. Processor/controller 104 may provide commands (e.g., specifying a physical memory address) to memory controller 106 to write values to or read values from an addressable portion of memory 108.

Memory array 108 may comprise one or more volatile or non-volatile memory devices including, for example, a memory array comprising volatile and non-volatile memory bitcells elements as described herein. Processor/controller 104, memory controller 106 and memory 108 may be formed as separate components or integrated together in a system-on-a-chip (SoC) along with other components not shown (e.g., sensors, user interface, I/O devices). Furthermore, processor/controller 104, memory controller 106 and memory array 108 may be formed from any one of several different process technologies including, for example, correlated electron material (CEM) processes discussed herein, complementary metal oxide semiconductor (CMOS) processes or other process used to form non-volatile memory bitcells or volatile memory bitcells, for example.

According to an embodiment, memory 108 may comprise volatile memory devices comprising volatile memory bitcells and non-volatile memory devices comprising non-volatile memory bitcells. Such volatile memory bitcells may comprise bitcells formed according to any one of several circuit structures for forming volatile memory bitcells such as SRAM bitcells, DRAM bitcells, just to provide a few examples. Such non-volatile memory bitcells may be formed according to any one of several non-volatile memory bitcells such as flash memory bitcells, correlated electron memory bitcells, phase change memory (PCM) bitcells, magnetic memory bitcells, just to provide a few examples. As described below in particular implementations, non-volatile and volatile memory bitcells formed in memory 108 may be integrated to enable copying of memory states between the non-volatile and volatile memory bitcells using any one of several different techniques.

FIG. 2 is a schematic diagram illustrating a process of copying states or transferring stored values between or among volatile memory bitcells and non-volatile memory bitcells, for example, within memory 108. FIG. 3A is a schematic diagram of a particular implementation in which memory states may be copied or stored values may be transferred between or among volatile memory bitcells 302 and non-volatile memory bitcells 304 over memory busses (e.g., fixed bit length memory busses) coupled between volatile memory bitcells 302 and non-volatile memory bitcells 304. In addition to volatile memory bitcells 302 and non-volatile memory bitcells 304, integrated circuit device 300 may comprise a shared data bus structure 300 and an external data port 322. According to an embodiment, shared data bus structure 330 may facilitate copy or transfer of stored values between or among volatile memory bitcells 302 and non-volatile memory bitcells 304. Shared data bus structure 330 may also facilitate transfer of stored data values between external data port 332 and either volatile memory bitcells 302 or non-volatile memory bitcells 304.

According to an embodiment, volatile memory bitcells 302 and non-volatile memory bitcells 304 may be formed in the single integrated circuit device 300 where volatile memory bitcells 302 may be formed in one or more volatile memory arrays and non-volatile memory bitcells 304 may be formed in one or more non-volatile memory bitcells.

Integrated circuit device 300 may comprise a plurality of external signal pins such as, for example, signal pins VM sel (volatile memory select), VM addr (volatile memory address), VM R/W (volatile memory read/write), VM cntl (volatile memory control) and VM clk (volatile memory clock). Likewise, integrated circuit device 300 may comprise a plurality of external signal pins such as, for example, signal pins NVM sel (non-volatile memory select), NVM addr (non-volatile memory address), NVM R/W (non-volatile memory read/write), NVM cntl (non-volatile memory control) and NVM clk (non-volatile memory clock). Integrated circuit device 300 may further comprise signaling pins 322 and 324 to at least in part form a single data port that is configurable to transfer data between an external device (not shown) and either volatile memory bitcells 302 or non-volatile memory bitcells 304. In addition, integrated circuit device 300 may comprise shared control signals such as a shared clock signal clk, shared address signal addr, among others. In a particular example, shared clock signal clk may control memory cycles (e.g., for read operations and/or write operations) applied to either volatile memory bitcells 302 or non-volatile memory bitcells, or both. Also, shared address signal addr may be used for accessing bitcells formed in either volatile memory bitcells 302 or non-volatile memory bitcells, or both.

In a particular implementation, integrated circuit device 300, including volatile memory bitcells 302, non-volatile memory bitcells 304 and shared bus structure 330, may be formed according to a digital circuit design within a single register transfer level (RTL) boundary defining a synchronous digital circuit (e.g., in terms of the flow of digital signals between or among registers and operations performed on such digital signals). In a particular implementation, the digital circuit design within the single RTL boundary may be defined according to a hardware description language (HDL) such as, for example, Verilog or VHDL, based on high-level representations of a circuit.

Integrated circuit 300 further comprise at least one volatile memory data bus 326 configurable to transfer data signals to volatile memory bitcells 302 in write operations and transfer stored values obtained from volatile bitcells 302 in read operations. Likewise, integrated circuit 300 further comprise at least one non-volatile memory data bus 328 configurable to transfer stored values to non-volatile bitcells 304 in write operations and transfer data obtained from non-volatile bitcells 304 in read operations. FIG. 3B is a schematic diagram of a specific implementation of integrated circuit device 300 shown in FIG. 3A including a specific implementation of shared data bus structure 330 and external data port 332. Here, a value for signal 306 provided to multiplexers 308 and 310 may indicate whether states are to be copied from volatile memory cells 302 to non-volatile memory cells 304, or from non-volatile memory cells 304 to volatile memory bitcells 302.

In this context, a “read operation” as referred to herein means an operation implemented by a circuit to detect a memory state of one or more bitcells. Further in this context, a “write operation” as referred to herein means an operation implemented by a circuit to place one or more bitcells in a particular memory state. For example, a write operation may comprise generation of a “programming signal” having particular properties (e.g., a voltage and/or current) which may be applied to one or more portions of a bitcell to place the bitcell in a particular memory state (e.g., a memory state that is detectable in a subsequent read operation).

In one embodiment, memory states or stored values of one or more volatile memory bitcells 302 may be copied or transferred to one or more non-volatile memory bitcells 304. In this context, values stored in one or more first memory bitcells may be “transferred” to one or more second memory bitcells by placing the one or more second memory bitcells in a particular memory state so as to store or represent, according to a particular mapping between stored values and memory states, the values stored in the one or more first memory bitcells. In the particular embodiment of FIG. 3, a transfer of stored values from one or more volatile memory bitcells 302 to one or more non-volatile memory bitcells 304 may comprise one or more read operations applied to the one or more volatile memory bitcells 302 to detect memory states of the one or more volatile memory bitcells 302 followed by one or more write operations applied to the one or more non-volatile memory bitcells 304. Similarly, a transfer of stored values from one or more non-volatile memory bitcells 304 to one or more volatile memory bitcells 302 may comprise one or more read operations applied to the one or more non-volatile memory bitcells 304 to detect memory states of the one or more non-volatile memory bitcells 304 followed by one or more write operations applied to the one or more volatile memory bitcells 302.

As shown in the specific implementation of FIG. 3B, external data port 332 may comprise external pins 322 and 324. In one embodiment, volatile memory data bus 326 may be configurable to transfer data received at external signal pins 322 to volatile memory bitcells 302 in a write operation according to a first state of multiplexer 308. Also, the least one volatile memory data bus 326 may be configurable for signaling to transfer data retrieved from volatile memory bitcells 302 in a read operation to external signal pins 324 according to a first state of multiplexer 312. Likewise in another embodiment, the least one non-volatile memory data bus 328 is configurable to transfer data received at external signal pins 322 to non-volatile memory bitcells 304 in a write operation according to a first state of multiplexer 310. Also, the least one non-volatile memory data bus 328 may be configurable to transfer data retrieved from non-volatile memory bitcells 304 in a read operation to external signal pins 324 according to a second state of multiplexer 312. Accordingly, by setting states of multiplexers 308, 310 and 312, integrated circuit device 300 may configure external data port 322 formed by external signal pins 322 and 324 to transfer data between an external device and either volatile memory bitcells 302 or non-volatile memory bitcells 304.

In another embodiment, the least one volatile memory data bus 326 and the least one volatile memory data bus 328 may be configured to transfer stored values between volatile memory bitcells 302 and non-volatile memory bitcells 304 independently of the data port formed by external signal pins 322 and 324. In one particular implementation, volatile memory bitcells 302, non-volatile memory bitcells 304, the least one volatile memory data bus 326 and the least one volatile memory data bus 328 may be configured to transfer stored values between volatile memory bitcells 302 and non-volatile memory bitcells independently of the data port formed by external signal pins 322 and 324 by application of a combination of signal conditions (e.g., including signal conditions affected by voltage levels, current levels, signal timing, etc.) to external signal pins (e.g., VM R/W, VM addr, VM sel, NVM R/W, NVM addr, NVM sel, etc.) of integrated circuit 300. Application of such voltages to external signal pins of integrated circuit 300 may, for example, place multiplexer 308 in a second state enabling the at least one volatile memory data bus 326 to receive stored values transferred from non-volatile memory bitcells 304 in a read operation. Here, received stored values transferred from non-volatile memory bitcells 304 in a read operation may be stored in volatile memory bitcells 302 in a subsequent write operation. Similarly, application of a combination of voltages to external signal pins of integrated circuit 300 may set multiplexer 310 in a second state enabling the at least one volatile memory data bus 310 to receive data transferred from volatile memory bitcells 302 in a read operation. Here, received stored values transferred from volatile memory bitcells 302 in a read operation may be stored in non-volatile memory bitcells 304 in a subsequent write operation.

In one implementation, volatile memory data bus 326 and non-volatile memory data bus 328 may have the same bus width (e.g., a byte or word) to transfer the same quantity of data or retrieved stored values on memory cycles. For example, buses 314 and 316 may comprise the same number of conductors, each conductor capable of transmitting a signal representing a single bit or symbol. In an alternative implementation, volatile memory data bus 326 and non-volatile memory data bus 328 may have different bus widths. For example, volatile memory data bus 326 may have a bus width that is an integer multiple the bus width of non-volatile memory data bus 328. In this example, multiplexer 310 may partition data transferred from volatile memory bitcells 302 in a single memory cycle for storage in non-volatile memory elements 304 in write operations over multiple memory cycles. Likewise, if non-volatile memory data bus 328 has a bus width that is an integer multiple larger than volatile memory data bus 326, multiplexer 308 may partition data transferred from non-volatile memory bitcells 304 in a single memory cycle for storage in non-volatile memory elements 302 in write operations over multiple memory cycles.

FIG. 4 illustrates timing of operations to copy states from volatile memory bitcells (e.g., volatile memory bitcells 302) to non-volatile memory bitcells (e.g., non-volatile memory bitcells 304). It should be understood that the particular timing of operations illustrated in FIG. 4 is merely an example of timing, and that other variations of timing may be employed without deviating from claimed subject matter. For example, particular variations may be directed to an active low wordline and/or pre signals, true-precharge versus post-charge schemes, etc. In an implementation, volatile and non-volatile memory bitcells may be accessed for write operations responsive to application of a voltage signal on a wordline to couple bitcell circuitry to one or more bitlines. A sense amplifier (not shown) may be maintained in an equalization mode until a wordline signal goes active. For example, such a sense amplifier may not become active until a certain signal level on a bitline is reached. Also, a write driver circuit (not shown) may be enabled prior to activation of a wordline.

In a first clock cycle, a voltage on a wordline coupled to one or more volatile memory bitcells (e.g., one or more volatile memory bitcells 302) may be raised to enable a read operation applied to the one or more VM bitcells. Signal VM WL active high may indicate that access of associated volatile memory bitcells is enabled if the associated signal is high and signal NVM WL active high may indicate that access of associated non-volatile memory bitcells is enabled if the associated signal is high. Signal VM Pre active low in a lower state may indicate precharging for bitlines to volatile memory bitcells (e.g., prior to signal VM WL active high is raised). Likewise, signal NVM Pre active low in a lower state may indicate precharging for bitlines to non-volatile memory bitcells (e.g., prior to signal NVM WL active high is raised). As shown, a leading edge of “Pre1” may precede a leading edge of the voltage signal on the wordline. Memory states or stored values detected in the read operation may then be copied or transferred to one or more bitcells in a write operation to one or more bitcells in non-volatile memory bitcells 304. Similar read and write operations may occur at subsequent clock cycles in pipeline fashion as shown. Accordingly, it may be observed that transfer of states from volatile memory bitcells (e.g., volatile memory bitcells 302) to non-volatile memory bitcells (e.g., non-volatile memory bitcells 304) may involve a latency.

Aspects of integrated circuit device 300 shown in FIGS. 3A and 3B may be implemented using features shown in FIGS. 5, 6, 8 and 12 as described below. FIG. 5 is a schematic diagram of a memory such as memory 108 comprising an array of volatile memory bitcells 504 and an array of non-volatile memory bitcells 506. Wordlines 508 may be used to access bitcells in the array of non-volatile memory bitcells 506 and wordlines 520 may be used to access bitcells in the array of volatile memory bitcells 504 for read and write operations. In this context, a “wordline” comprises a conductor for transmitting a signal to select a particular bitcell or group of bitcells to be accessed in a read operation or a write operation. In a particular example implementation, a voltage of a signal on a wordline may be raised or lowered to select or deselect a particular bitcell or group of bitcells to be connected to a corresponding bitline or group of bitlines during a read or write operation. It should be understood, however, that this is merely an example of a wordline and that claimed subject matter is not limited in this respect.

To enable copying of memory states (or transfer of corresponding stored values) between volatile memory bitcells in array of volatile memory bitcells 504 and non-volatile memory bitcells 506, a particular wordline 508 may be used to access one or more bitcells in the array of non-volatile memory bitcells 506 and a corresponding wordline 520 may be used to access and one or more bitcells in the array of volatile memory bitcells 504.

In one embodiment, decoder circuit 510 may comprise a “shared decoder circuit” in that decoder circuit 510 may, among other things, assert voltage signals on wordlines 520 to access bitcells in the array of volatile memory bitcells 504 and assert voltage signals on wordlines 508 to access bitcells in the array of non-volatile memory bitcells 506. In an alternative embodiment, decoder circuit 510 may assert voltage signals on wordlines 520 to access bitcells in the array of volatile memory bitcells 504 and a second, optional decoder 512 may assert voltage signals on wordlines 508 to access bitcells in the array of non-volatile memory bitcells 506. According to an embodiment, assertion of a wordline 520 may connect corresponding bitcells in volatile memory 504 to bitlines (not shown) connected to I/O circuitry 516. I/O circuitry 516 may comprise sense amplifier circuits (not shown) for detecting memory states of volatile memory bitcells connected to bitlines in read operations. I/O circuitry 516 may also comprise write driver circuits (not shown) to generate programming signals to affect a memory state of volatile memory bitcells connected to bitlines in write operations. Likewise, I/O circuitry 518 may comprise sense amplifier circuits (not shown) for detecting memory states of non-volatile memory bitcells connected to bitlines in read operations. I/O circuitry 518 may also comprise write driver circuits (not shown) to generate programming signals to affect a memory state of non-volatile memory bitcells connected to bitlines in write operations. In this context, a “bitline” comprises a conductor that is connectable to at least a portion of a bitcell circuit during a write operation to transmit a signal altering a memory state of the bitcell circuit, or during a read operation to transmit a signal representative of a memory state of the bitcell circuit. According to an embodiment, bus 550 coupled between I/O circuitry 516 and 518 may facilitate copying of memory states (or transfer of corresponding stored values) between one or more volatile memory bitcells 504 and non-volatile memory bitcells 506. For example, bus 550 may comprise an address portion identifying target bitcells to be affected by a write operation and a data portion (e.g., having a data bus width) to transmit one or more signals indicative of memory states obtained in a read operation to be written to the target bitcells in the write operation.

In a particular implementation, a data bus width and word address width for accessing bitcells in array of volatile memory bitcells 504 (e.g., at I/O circuitry 516) may be the same as a data bus width and word address width for accessing bitcells in array of non-volatile memory bitcells 506 (e.g., at I/O circuitry 518). However, corresponding wordlines 520 and 508 may be connected through buffers 514 formed between portions of bitcells in the array of volatile memory bitcells 504 and the array of non-volatile memory bitcells 506. In an implementation, to enable decoder circuit 510 to operate as a shared decoder circuit as discussed above, buffers 514 may re-shape voltage signals generated by decoder 510 on corresponding wordlines 520 to be applied to wordlines 508 for accessing bitcells in non-volatile memory array 506. Here, for example, an access signal may be applied to a wordline 508 in response to decoder 510 applying an access signal to a wordline 520 coupled to the bitline 508 through a buffer 514. In an alternative implementation, an optional decoder 512 may generate signals on wordlines 508 to access bitcells in array of non-volatile memory bitcells 506 while decoder 510 generates signals on wordlines 520 to access bitcells in array of volatile memory bitcells 504. Buffers 514 may also perform a latching function to implement pipelining of operations between bitcells of the array of volatile memory bitcells 504 and bitcells of the array of non-volatile memory bitcells 506. For example, a buffer 514 may affect wordline signal to enabling pipelining of read and write operations to copy memory states between bitcells of the array of volatile memory bitcells 504 and bitcells of the array of non-volatile memory bitcells 506.

As discussed above, in one embodiment, memory states may be copied (or corresponding stored values may be transferred) between bitcells of the array of volatile memory bitcells 504 and bitcells of the array of non-volatile memory bitcells 506. In a particular implementation, states of bitcells in array of volatile memory bitcells 504 coupled to a particular wordline 520 may be copied or written to bitcells in array of non-volatile memory bitcells 506 coupled to a particular wordline 508 (coupled to the particular wordline 520 through a buffer 514). Timing of such a transaction be illustrated in FIG. 7A according to an embodiment. Here, a signal R1 may be asserted on a wordline 520 to be applied to selected bitcells in array of non-volatile memory bitcells 504 for a read operation following a leading edge of a clock pulse in a first clock cycle. During the read operation, circuitry in I/O circuitry 516 connected to the selected bitcells by bitlines may detect memory states of the selected bitcells. Following detection of states in selected bitcells in the array 504 from the read operation, a signal W1 may then be asserted on a wordline 508 to be applied to bitcells in array of non-volatile memory bitcells 506 at a trailing edge of the clock pulse in the first clock cycle. Here, bitcells in array of non-volatile memory bitcells 506 may be accessed for a write operation to write or copy detected states of the selected bitcells in the array of volatile memory bitcells 504 to the accessed bitcells in array of non-volatile memory bitcells 506. As pointed out above, a buffer 514 in wordline 508 may provide a latch at the boundary between the bitcells coupled to wordline 508 in array 504 accessed for the read operation and the bitcells coupled to the wordline 508 in the array 506 accessed for the write operation. This may allow a falling edge of voltage on wordline 520 coupled to array 504 and the Pre signal (restoring the bitlines of the VM) may also fall. In other implementations, a buffer 514 may comprise level shifter circuit or a latch circuit to raise or lower a voltage on an associated wordline. For example, buffer 514 comprising a level shifter circuit may respond to a first voltage on a wordline 520 (enabling access to selected volatile memory bitcells of array 504) by applying a second, different voltage on a wordline 508 (enabling access to selected non-volatile memory bitcells of array 506).

As may be observed from FIG. 7A, a leading edge of wordline signal W1 for access of bitcells in array of non-volatile memory bitcells 506 may occur at a trailing edge of a clock pulse in an immediately preceding clock cycle. A VM Pre active low signal may be asserted low after the wordline signal R1 is de-asserted or returns low following the write operation occurring during assertion of wordline voltage signal R1.

As pointed out above in connection with FIGS. 3A and 3B, volatile memory bus 326 and non-volatile memory bus 328 may have different bus widths (e.g., wherein one bus width is an integer multiple of the other bus width). Similarly, bus 550 may comprise a bus width to access volatile memory array 504 that is different from a bus width to access non-volatile memory array 506. In a particular numerical example implementation, non-volatile memory array 506 may be accessed with a bus width of 64-bits while volatile memory array 504 may be accessed with a bus width of 16-bits such that 64-bits may be transferred between non-volatile memory array 506 and volatile memory array 504 on four memory cycles of volatile memory array 504 and a single memory cycle of non-volatile memory array 506. It should be understood that this is merely one particular numerical example and that different width may be implemented (e.g., with non-integer widths, bus width to access volatile memory array 504 being wider than a bus width to access non-volatile memory array 506) without deviating from claimed subject matter. In this particular example, I/O circuitry 516 may comprise a 4×1 column multiplexer (not shown) to facilitate transfer of four 16-bit words of volatile memory array 504 between a single 64-bit word of non-volatile memory array 506. Similarly, there may be a data in port (not shown) in I/O circuitry 518. This data in port may have a width of 16-bits, or may have a width of 64-bits. To support such a transfer of 64-bits in a single transaction of non-volatile memory array 506 and four transactions of volatile memory array 504, a corresponding buffer 514 may maintain or latch a particular access signal on a wordline 508 at an active state (e.g., at a constant voltage) while an access signal on a wordline 520 cycles between precharge and active phases for four transactions.

To support transfer of stored values between volatile memory bitcells 504 and non-volatile memory bitcells 506 in the case where the bus width to access non-volatile memory bitcells 506 is four times that of the bus width to access volatile memory bitcells 504, a buffer 514 may further comprise a latch to maintain an access signal on a decoded wordline 508 for selected non-volatile memory bitcells 506. For example, such an access signal may be maintained on a decoded wordline 508 until data comprising a complete non-volatile bus width has been read from selected volatile memory bitcells 504. In the above example in which a bus width to access volatile memory bitcells 504 is 16-bits and a bus width to access non-volatile memory bitcells 506 is 64-bits, sixty four bits of data may be stored in volatile memory bitcells 504 on the same wordline 520 or row (and thus a decoder address may be unchanged) but across a column address width of 4-bits. If column addresses of volatile memory bitcells 504 are to be decoded according to an eight to one encoding scheme, the column multiplexer may select one of eight columns while a wordline 520 is selected. In three subsequent accesses of volatile memory bitcells 504 on the selected wordline 520, the same row address may be accessed while a column address may cycle from bit 0 to bit 1 . . . ending on bit 3. In this manner accesses of volatile memory bitcells 504 may occur in four access cycles while using the same row address. For each such an access cycle, 16-bits may be read from or written to via bus 550. While a row address may remain the same, a buffer 514 (which may comprise a latch as discussed above) may maintain a value of decoded wordline 508 so that the row is selected for the selected volatile memory bitcells 506.

In one implementation for accessing selected non-volatile memory bitcells 506 in a write operation in connection with four corresponding cycles to access selected volatile memory bitcells 504, selected non-volatile memory bitcells 506 may be accessed through write operations in four different cycles. In an alternative implementation, a write buffer in I/O circuitry 516 (not shown) may accumulate a full 64-bits prior to enabling a wordline 506 for a write operation applied to the selected non-volatile memory bitcells 506. Controls for this wordline 508 may comprise a combination of self-timed and clock signals depending upon a particular implementation. Therefore, features of a buffer 514 may support a case in which a bus width to access volatile memory bitcells 504 and a bus width to access non-volatile memory bitcells 506 are not equal.

In the particular implementations of FIGS. 5 and 6, volatile memory bitcells are shown to be adjacent to a decoder circuit and non-volatile memory bitcells are shown to be (or flanked by the decoder and non-volatile memory bitcells). In an alternative embodiment to the implementations of FIGS. 5 and 6, non-volatile memory bitcells may be formed to be adjacent to (and flanked by) both a decoder circuit and volatile memory bitcells. Here, a buffer circuit may also be used to reshape an access signal applied to a wordline connected to the volatile memory bitcells (e.g., instead of reshaping an access signal applied to a wordline connected to the non-volatile memory bitcells).

FIG. 6 is a schematic diagram of an alternative embodiment in which wordlines 608 may be used to access bitcells in array of volatile memory bitcells 604 and the array of non-volatile memory bitcells 606 for read and write operations. As in the embodiment of FIG. 5 in which a voltage on a single wordline may be asserted by decoder 510 to access non-volatile memory bitcells 506 and volatile memory bitcells 504 (e.g., through a corresponding buffer 514), decoder 610 may comprise a “shared decoder circuit” to assert a voltage signal on a single wordline 608 to access bitcells in array of volatile memory bitcells 604 and the array of non-volatile memory bitcells 606. Also, decoder 610 may employ the same word address bus for accessing bitcells in array of volatile memory bitcells 604 and the array of non-volatile memory bitcells 606. Bus 650 coupled between I/O circuitry 616 and 618 may facilitate copying of memory states (or transferring of corresponding stored values) between one or more volatile memory bitcells 604 and non-volatile memory bitcells 606. For example, bus 650 may comprise an address portion identifying target bitcells to be affected by a write operation and a data portion (e.g., having a data bus width) to transmit one or more signals indicative of memory states obtained in a read operation to be written to the target bitcells in the write operation.

In an implementation, data bus widths of bus 650 between I/O circuitry 616 and I/O circuitry 618 may be same. Alternatively, a data bus width at I/O circuitry 616 may be an integer multiple of a bus width at I/O circuitry 618, or a data bus width at I/O circuitry 618 may be an integer multiple of a bus width at I/O circuitry 616. For simplicity of this discussion, data bus widths at I/O circuitry 616 and I/O circuitry 618 are presumed to be the same (e.g., same number of bits or bytes). It should be understood, however, that data bus widths at I/O circuitry 616 and I/O circuitry 618 may be different without deviating from claimed subject matter. For example, if I/O circuitry 616 has a data bus width that is an integer multiple of a data bus width of I/O circuitry 618, transfer of values between I/O circuitry 616 and I/O circuitry 618 may entail a single access cycle for I/O circuitry 616 and the integer multiple access cycles for I/O circuitry 618. In other implementations, however, use of read or write masks may enable a bus width of I/O circuitry 616 that is not necessarily an integer multiple of bus width of I/O circuitry 618.

In the particular embodiment of FIG. 6, bitcells in array of volatile memory bitcells 604 and bitcells in array of non-volatile memory bitcells 606 may be simultaneously accessed by assertion of a single wordline 608. In other words, a single wordline 608 may be used to access corresponding bitcells in both arrays 604 and 606 without a buffer (e.g., buffer 514) connecting a first bitline to access bitcells in array 604 and a second bitline in to access bitcells in array 606. Employing the same wordline decoding scheme for accessing bitcells in array of volatile memory bitcells 604 and the array of non-volatile memory bitcells 606, the embodiment of FIG. 6 may enable a tighter coupling between bitcells in array 604 and bitcells in array 606. In particular implementations, embodiments of FIG. 5 or FIG. 6 may enable a read modify write implementation that would potentially shorten a latency to copy memory states between non-volatile memory bitcells (e.g., of array 504 or 604) and volatile memory bitcells (e.g., of array 506 or 606).

In some implementations, copying or transferring a quantity of multiple memory states or stored values (e.g., a “packet” such as byte or word) between non-volatile memory bitcells and volatile memory bitcells may entail a minimum of two memory cycles (e.g., two memory clock cycles). For example, a read operation on a portion of a first memory to detect memory states may consume a first memory cycle and a subsequent operation to write the detected memory states to a portion of a second memory may consume a second memory cycle. Accessing a first memory to read and a second memory to write may entail additional time to restore bitline voltages.

With a shared wordline 608 as illustrated in FIG. 6, it may be possible to assert a voltage signal on the shared wordline 608 to perform a read operation to detect memory states in a first array, and then maintain the voltage signal continuously while performing an operation to write the detected memory states to a second array. Here, while shared wordline 608 is asserted to access bitcells in array of volatile memory bitcells 604 and bitcells in array of non-volatile memory bitcells 608, I/O circuitry 616 and I/O circuitry 618 may perform read and write operations within the same memory cycle. If copying memory states from selected bitcells in array of volatile memory bitcells 604 to selected bitcells in array of non-volatile memory bitcells 606, in the same memory clock cycle sense amplifiers of I/O circuitry 616 may detect memory states of the selected bitcells in the array of volatile memory bitcells 604, and write driver circuits of I/O circuitry 618 may apply programming signals to the selected bitcells of array of non-volatile memory bitcells 606 to write the detected memory states. Similarly, if copying or transferring memory states or stored values from selected bitcells in array of non-volatile memory bitcells 606 to selected bitcells in array of volatile memory bitcells 604, bus 650 in the same memory clock cycle sense amplifiers of I/O circuitry 618 may detect memory states of the selected bitcells in the array of non-volatile memory bitcells 606 and write driver circuits of I/O circuitry 616 may apply programming signals to the selected bitcells of array of volatile memory bitcells 604 to write values corresponding to the detected memory states.

According to an embodiment, voltages of bitlines connecting bitcells in array of volatile memory bitcells 604 to I/O circuitry 616 and connecting bitcells in array of non-volatile memory bitcells 606 to I/O circuitry 618 may be restored following read or write operations. As pointed out above in an implementation, read and write operations to copy memory states between volatile memory bitcells 604 and non-volatile memory bitcells may occur in a single clock cycle. Accordingly, this particular two-part access procedure (read operation to detect a memory state of selected bitcells and write operation to place selected bitcells in the detected memory state) may be performed in a single memory access cycle. Accordingly, voltages on bitlines connecting affected bitcells and I/O circuitry 616 and 618 may be restored in a single period following the single clock cycle to perform the read and write operations. In the timing diagram of FIG. 7B illustrating a particular example, a read operation to detect memory states of a first memory followed by an operation to write the detected memory states to a second memory may occur in a cycle time that is 1.5 times that of a memory cycle for accessing non-volatile memory bitcells in array 604 (e.g., assuming that a duration for a read operation to detect memory states in bitcells of array 604 and a duration for a subsequent operation to write the detected memory states to bitcells in array 606 are approximately the same). Here, FIG. 7B shows that a voltage signal on a wordline connected to non-volatile memory bitcells and volatile memory bitcells is raised following a leading edge of a first clock pulse and lowered following a leading edge of the next clock pulse. While the voltage signal on the wordline is raised, a read operation R1 detects memory states of volatile memory bitcells and a subsequent write operation W1 may write values corresponding to the detected states to non-volatile memory bitcells. Bitline voltages for read operation R1 and write operation W1 may then be restored in a single memory access cycle following a lowering of the voltage signal on the wordline. In other embodiments, a duration to access bitcells in a non-volatile memory array may be longer than a duration to access bitcells in a volatile memory array. Accordingly, the particular implementation of FIG. 7A may reflect a trade-off between a slower cycle time for a smaller area and power.

The specific implementation of FIG. 6 shows that volatile memory bitcells 604 are configured to be between decoder 610 and non-volatile memory bitcells 606, with decoder 610 and non-volatile memory bitcells 606 flanking volatile memory bitcells 604. In an alternative implementation, decoder 610 may be configured to be placed between two physically separate portions of volatile memory bitcells 604 flanking the decoder 610, and two physically separate portions of non-volatile memory bitcells 606 flanking the physically separate portions of volatile memory bitcells 604 in a “butterfly configuration.” Here, decoder circuit 610 may apply access signals directly to wordlines connected to selected bitcells in either separate portion of volatile memory bitcells 604. Also, an access signal applied to a wordline connected to a particular separate portion of volatile memory bitcells 604 may access bitcells in a particular portion of non-volatile memory bitcells 606.

In another alternative implementation (also not shown), volatile memory bitcells 604, non-volatile memory bitcells 606, decoder 610, I/O circuitry 616 and I/O circuitry 618 may be further configured to be in a “four quadrant” butterfly configuration. Here, in the aforementioned butterfly configuration, each physically separate portion of volatile memory bitcells 604 may be further partitioned or bifurcated into two additional portions such that a portion of I/O circuitry 616 may be formed between the two portions of the physically separate portion of volatile memory bitcells 604. Similarly, each physically separate portion non-volatile memory bitcells 606 may be further partitioned or bifurcated into two additional portions such that a portion of I/O circuitry 618 may be formed between the two portions of the physically separate portions of non-volatile memory bitcells 606.

In yet another alternative implementation (also not shown), decoder 610 may be configured to be between volatile memory bitcells 604 and non-volatile memory bitcells 606 such that volatile memory bitcells 604 and non-volatile memory bitcells 606 flank decoder 610 in an “asymmetric butterfly configuration.” Here, decoder circuit 610 may apply signals directly to wordlines connected to selected bitcells in volatile memory access bitcells 604 or to selected non-volatile memory bitcells 606.

FIG. 8 is a schematic diagram of a memory system integrating volatile memory bitcells and non-volatile memory bitcells according to an embodiment. Here, volatile memory bitslice 804 may comprise one or more volatile memory bitcells interleaved with non-volatile memory bitcells of non-volatile memory bitslice 806. In an implementation, corresponding volatile memory bitcells of volatile memory bitslice 804 and adjacent or local non-volatile memory bitcells of non-volatile memory bitslice 806 may be accessed responsive to a single voltage asserted on a wordline as discussed above in connection with FIGS. 5, 6, 7A and 7B. For example, memory states or stored values in one more bitcells of volatile memory bitslice 804 may be copied or transferred to one or more bitcells of non-volatile memory bitslice 806 based on a read operation performed at I/O circuitry 816 to detect memory states of the one or bitcells of array of volatile memory bitcells 804, and a write operation performed by I/O circuitry 818 to write values corresponding to the one or more detected memory states.

In some implementations, transferring stored values between volatile and non-volatile memory bitcells (e.g., as discussed above in connection with FIGS. 5 and 6) may increase or stress memory bus requirements. The particular implementation of FIG. 8, on the other hand, may enable transfer of stored values between adjacently formed volatile and not-volatile memory bitcells without accessing a memory bus coupled between a uniform array of volatile memory bitcells (e.g., volatile memory array 504 or 604) and a uniform array of non-volatile memory bitcells (e.g., non-volatile memory array 506 or 606). As shown in FIG. 8, stored values may be transferred between one or more bitcells of bitslice of volatile memory bitcells 804 and one or more bitcells of bitslice of non-volatile memory bitcells 806.

In one embodiment, I/O circuitry 816 may comprise a read circuit connected to the one or more common first bitlines to detect a value stored in the in a selected volatile memory bitcell of volatile memory bitslice 804 and I/O circuitry 818 may comprise a write driver circuit connected to the one or more common second bitlines to generate a programming signal to store a value corresponding to the detected value in a selected non-volatile memory bitcell of non-volatile memory bitslice 806. Here, values stored in one or more volatile memory bitcells of volatile memory bitslice 804 may be transferred to one or more non-volatile memory bitcells of non-volatile memory bitslice 806 with a simple read-modify-write transaction and without accessing an external bus device. Similarly, I/O circuitry 818 may comprise a read circuit connected to the one or more common first bitlines to detect a value stored in the in a selected non-volatile memory bitcell of non-volatile memory bitslice 806 and I/O circuitry 816 may comprise a write driver circuit connected to the one or more common second bitlines to generate a programming signal to store a value corresponding to the detected value in a selected volatile memory bitcell of volatile memory bitslice 804. Here, values stored in one or more volatile memory bitcells of non-volatile memory bitslice 806 may be transferred to one or more non-volatile memory bitcells of volatile memory bitslice 804 with a simple read-modify-write transaction without accessing an external bus device.

In one implementation, bitslice of volatile memory bitcells 804 or bitslice of non-volatile memory bitcells 806 may comprise a single bit “column slice” of bitcells connected to a single bitline and selectable by a wordline, or multiple bits coupled to multiple corresponding bitlines selectable by a single wordline. In one embodiment in which either bitslice 804 or bitslice 806 comprises multiple bitcells accessible by a single wordline, a multiplexer (not shown) may be used to connect a bitline of a selected bitcell (e.g., selected among multiple bitcells accessible by a wordline), to read a circuit or a write circuit to effect a transfer of a stored value between the selected bitcell and another bitcell as discussed above. In a particular implementation, a first multiplexer may facilitate transfer of stored values from bitslice 804 to bitslice 806, and a second multiplexer may facilitate transfer of stored values from bitslice 806 to bitslice 804. For example, widths of the first and second multiplexers may not be necessarily equal if a number of bitlines connected to bitslice 804 is not equal to a number of bitlines connected to bitslice 806.

FIG. 9 is a schematic diagram illustrating an addressing scheme integrating volatile memory bitcells and non-volatile memory bitcells according to an embodiment. Such an addressing scheme may be implemented, for example, in a decoder circuit (e.g., decoder circuit 510 or decoder 610) for selecting bitcells to be accessed for a read or write operation. In an implementation, a physical address A may range from a value n through o. For example, addressable portions (e.g., fixed length words) in an array of volatile memory bitcells (e.g., arrays of volatile memory bitcells 504 or 604) may be accessed according to addresses in a range of values n to m. Similarly, addressable portions in an array of non-volatile memory bitcells (e.g., arrays of non-volatile memory bitcells 506 or 606) may be accessed according to addresses in a range of values above m to o.

According to an embodiment, physical address A ranging from value n through o may represent an encoded address or a decoded address. For example, if an encoded address bus is written as A<0:5>, six bits <0>, <1> . . . <5> may be decoded or expanded into 64 signals representing an address, say WL<0:63>. In a particular implementation, an address space WL<0:63> may be partitioned into volatile memory and non-volatile memory components. For example, WL<0:15> may be dedicated for accessing non-volatile memory components and WL<16:63> may be dedicated for accessing volatile memory components.

In another embodiment in which physical address A ranging from value n through o represents a decoded address, overlapping portions of a bus may be used to access either volatile memory or non-volatile memory. For example, address WL<0:31> may access NVM while address WL<16:63> may access VM. Address WL<16:31>, while accessed, may be used to read/write from/into both VM and NVM.

Embodiments of FIGS. 5 and 6 are directed to a coupling of arrays of volatile memory bitcells and non-volatile memory bitcells to facilitate copying of memory states (or transferring stored values) between the arrays of volatile memory bitcells and non-volatile memory bitcells. According to an embodiment, FIG. 10 is a schematic diagram integrating only a portion of array of volatile memory bitcells 1004 with array of non-volatile memory bitcells 1006. In this particular implementation, array of volatile memory bitcells 1004 may be formed as a cache memory in a computing platform partitioned into cache “ways” Way0, Way1, Way2 and Way3. As shown, array of non-volatile memory bitcells 1006 may be coupled to Way0 to facilitate copying of memory states between non-volatile memory bitcells 1006 and Way0. In an alternative implementation as shown in FIG. 11, only a portion of array of non-volatile memory bitcells 1106 is coupled with array of non-volatile memory bitcells 1104.

In the particular implementations of FIGS. 10 and 11, addresses for accessing volatile or non-volatile memory bitcells may be subsets of one another. For example, as shown in FIG. 10, addresses for accessing volatile memory 1004 (e.g., for portions of cache) may be a subset of addresses for accessing portions of non-volatile memory 1006. Similarly, as shown in FIG. 11, addresses for accessing non-volatile memory 1106 may be a subset of addresses for accessing portions of volatile memory 1104.

Embodiments discussed above in connection with FIGS. 5 and 6 are directed to copying memory states between non-volatile memory bitcells and volatile memory bitcells by asserting a single access signal on a wordline to access non-volatile memory bitcells and volatile memory bitcells (e.g., by asserting a single access signal on a wordline 608 connected to selected bitcells in volatile memory array 604 and selected bitcells in non-volatile memory array 606, or by asserting a single access signal on a wordline 504 to access selected bitcells of volatile memory array 504 such that a corresponding wordline 508 is asserted through a buffer 514 to access selected bitcells of non-volatile memory array 506). FIG. 12 is a schematic diagram illustrating an interleaving of volatile memory bitcells and non-volatile memory bitcells according to an embodiment. In particular, an array of non-volatile memory bitcells 1206 is coupled to two arrays of volatile memory bitcells 1204 and 1224. According to an embodiment, memory states may be copied (or corresponding stored values transferred) between array of non-volatile memory bitcells 1206 and either array of volatile memory bitcells 1204 or array of volatile memory bitcells 1224.

In one implementation, bitcells in array of volatile memory bitcells 1204 may be accessed for a read operation responsive to a voltage signal applied to a first wordline 1220 such that memory states are detected by I/O circuitry 1216. Bitcells in array of non-volatile memory bitcells 1206 may be subsequently accessed for a write operation responsive to a second wordline signal applied to a wordline 1208 to copy memory states detected by I/O circuitry 1216. For example, the wordline 1208 may be coupled to the wordline 1220 through at least a tri-state buffer 1230 and possibly an optional flip-flop circuit 1234. Similar actions may be performed to copy memory states detected in bitcells of array of volatile memory bitcells 1224 to bitcells in array of non-volatile memory bitcells 1206 facilitated by tri-state buffer 1232 and possibly flip-flop circuit 1236, or to copy memory states detected in bitcells of array of non-volatile memory bitcells 1206 to bitcells of either array of volatile memory bitcells 1204 or 1224.

According to an embodiment, tristate buffers 1230 may isolate array of volatile memory bitcells 1204 while copying memory states between array of non-volatile memory bitcells 1206 and array of volatile memory bitcells 1224. For example, during read and write operations to copy memory states between bitcells in array of non-volatile memory bitcells 1206 and array of volatile memory bitcells 1224, tristate buffers 1230 may disconnect wordlines 1220 from wordlines 1208 and tristate buffers 1232 may connect selected wordlines 1208 and 1222. Similarly, tristate buffers 1232 may isolate array of volatile memory bitcells 1224 while copying memory states between array of non-volatile memory bitcells 1206 and array of volatile memory bitcells 1204. For example, during read and write operations to copy memory states between bitcells in array of non-volatile memory bitcells 1206 and array of volatile memory bitcells 1204, tristate buffers 1230 may disconnect wordlines 1220 from wordlines 1208 and tristate buffers 1232 may connect selected wordlines 1208 and 1222.

As pointed out above, the embodiment of FIG. 12 may include optional flip-flop circuits 1234 and 1236 to enable pipelining of operations to copy memory states between bitcells in array of non-volatile memory bitcells 1206 and bitcells in array of volatile memory bitcells 1204 or 1224. For example, during a read operation to detect memory states of selected bitcells in array of bitcells 1204, a rising edge voltage on a wordline 1220 may couple the selected bitcells to I/O circuitry 1216. In response to the rising edge voltage on wordline 1220, a flip-flop circuit 1234 may apply a voltage signal on a wordline 1208 to decouple corresponding bitcells in array of non-volatile memory bitcells 1206 from I/O circuitry 1218. Following completion of the read operation, a falling edge of a voltage on the wordline 1220 applied to the flip-flop circuit 1234 may change the voltage on the wordline 1208 so as to couple the corresponding bitcells array of non-volatile memory bitcells 1206 from I/O circuitry 1218 for a write operation.

According to an embodiment, tristate buffers 1230 and 1232 may additionally comprise level shifters to enable application of different wordline voltages to access volatile memory (on wordlines 1220 or 1222) and to access non-volatile memory on wordlines 1208. Also, non-volatile memory bitcells of 1206 may be accessed by signals originating at either decoder 1210 or decoder 1212. If transferring values between volatile memory bitcells 1204 and non-volatile memory bitcells 1206, for example, a voltage on a wordline 1208 may be affected to access non-volatile memory bitcells 1206 responsive to decoder circuit 1210. Likewise, if transferring values between volatile memory bitcells 1224 and non-volatile memory bitcells 1206, for example, a voltage on a wordline 1208 may be affected to access non-volatile memory bitcells 1206 responsive to decoder circuit 1212.

FIG. 13A is a schematic diagram of a bitcell circuit 1300 comprising volatile memory elements and non-volatile memory elements according to an embodiment. Bitcell circuit 1300 comprises two non-volatile memory elements NV₁ and NV₂, and two volatile memory elements formed in part by PFETs P1 and P2. Non-volatile memory elements NV₁ and NV₂ may be formed using any one of several different types of non-volatile memory devices including, for example, flash memory devices, correlated electron memory devices, phase change memory (PCM) devices, magnetic memory devices, just to provide a few examples. A voltage source 1306 is coupled to first terminals of non-volatile memory elements NV₁ and NV₂, and PFETs P1 and P2. Multiplexers 1302 and 1304 may selectively couple either second terminals of non-volatile memory elements NV₁ and NV₂ or second terminals of PFETs P1 and P2 to latch nodes T and C depending on whether bitcell circuit 1300 is to operate in a volatile memory mode or a non-volatile memory mode. In a particular implementation, multiplexers 1302 and 1304 may selectively couple either second terminals of non-volatile memory elements NV₁ and NV₂ or second terminals of PFETs P1 and P2 to latch nodes T and C responsive to a state of a selection signal SEL. In one example, selection signal SEL may comprise a single signal input. In another example, selection signal SEL may comprise multiple signal inputs on a bus In a particular implementation, to enable appropriate transition to a volatile memory mode, multiplexers 1302 and 1304 may transition connection of latch nodes T and C from non-volatile memory elements NV₁ and NV₂ to second terminals of PFETs P1 and P2 such that latch nodes T and C may be connected to second terminals of PFETs P1 and P2 prior to disconnection of latch nodes T and C from non-volatile memory elements NV₁ and NV₂.

In one implementation, bitcell circuit 1300 may be employed in a device that transitions between a powered up and powered down state. Here, it may be desirable to a preserve a particular memory state of volatile memory elements as the device transitions to a powered down state such that the particular memory state may be restored at a future time when the device transitions back to a powered up state. For example, as the device transitions to a powered down state, it may be desirable to copy a current state of non-volatile memory elements to non-volatile memory elements NV₁ and NV₂. This may comprise, for example, read operations to detect states of the non-volatile memory elements followed by operations to write the detected states to non-volatile memory elements NV₁ and NV₂.

In another example, as the device transitions from a powered down state to a powered up state, it may be desirable to copy or transfer a current state of non-volatile memory elements NV₁ and NV₂ to non-volatile memory elements. This may comprise, for example, read operations to detect states of the non-volatile memory elements NV₁ and NV₂ followed by write operations to store the detected states to volatile memory elements.

According to an embodiment, bitcell circuit 1300 may transfer or copy values stored in non-volatile memory elements NV₁ and NV₂ to non-volatile memory elements during a power up operation (e.g., power on reset). This may occur, for example, while FETs N1 and N2 are open (e.g., while signal WL is low) such that the bitcell circuit is disconnected from bitlines BL and BL′. In a particular implementation, non-volatile memory elements NV₁ and NV₂ may store a value, parameter, condition or symbol as a complementary resistance state or impedance state. In other words, non-volatile memory elements NV₁ and NV₂ may store a first value, parameter, condition or symbol by having NV₁ in a high impedance/resistance state and NV₂ in a low impedance/resistance state, and may store a second value, parameter, condition or symbol by having NV₁ in a low impedance/resistance state and NV₂ in a high impedance/resistance state. As power is applied at voltage source 1306, multiplexer 1302 may connect a terminal of NV₁ to node T (while disconnecting PFET P1 from node T) and multiplexer 1304 may connect a terminal of NV₂ to node C (while disconnecting PFET P2 from node C). Multiplexer 1302 may then connect PFET P1 to node T and multiplexer 1304 may connect PFET P2 to node C. If NV₁ is in a high impedance/resistance state and NV₂ is in a low impedance/resistance state (e.g., to store the first value, parameter, condition or symbol), node C may be placed at a higher voltage than node T, causing PFET P1 to be open and causing PFET P2 to be closed as part of a latch circuit including PFETs P1 and P2, and NFETs N3 and N4. Conversely, if NV₁ is in a low impedance/resistance state and NV₂ is in a high impedance/resistance state (e.g., to store the second value, parameter, condition or symbol), node C may be placed at a lower voltage than node T, causing PFET P1 to be closed and causing PFET P2 to be open. Following connection of PFETs P1 and P2 to nodes T and C, respectively, multiplexer 1302 may disconnect non-volatile memory element NV₁ from node T and multiplexer 1304 to disconnect NV₂ from node C, allowing bitcell circuit 1300 to operate as a volatile memory bitcell. In a particular implementation, while bitcell circuit is operating in non-volatile mode (while non-volatile memory elements NV₁ and NV₂ are connected to nodes T and C, respectively) voltages at nodes T and C may represent values stored in non-volatile memory elements NV₁ and NV₂, respectively. Similarly, while bitcell circuit is operating in a volatile mode (while non-volatile memory elements NV₁ and NV₂ are disconnected from nodes T and C, respectively) voltages at nodes T and C may represent values stored in volatile memory elements. Further, voltages at nodes T and C represent values that may be transferred between non-volatile memory elements NV₁ and NV₂, and volatile memory elements formed in part by PFETs P1 and P2.

FIG. 13B is a schematic diagram of a specific implementation of bitcell circuit 1300 shown as bitcell circuit 1310 in which multiplexer 1302 is implemented as NFET N5 and PFET P3, and multiplexer 1304 is implemented as NFET N6 and PFET P4. Here, bitcell circuit 1310 may be switchable between operation in a volatile memory mode by lowering a voltage of signal power-on-reset (POR) and operation in a non-volatile memory mode by raising a voltage of signal POR.

In the alternative implementation of FIG. 13C, a bitcell circuit 1312 may remove PFETs P3 and P4 from bitcell circuit 1310 of the particular implementation of FIG. 13B while maintaining NFETs N5 and N6 to selectively connect non-volatile memory element NV₁ to node T and non-volatile memory element NV₂ to node C responsive to a signal POR signal applied to gates of NFETs N5 and N6. However, PFET P1 remains connected to node T and PFET P2 remains connected to node C. Here, a voltage of signal POR may be may be raised following application of power at voltage source 1306. In an embodiment, the voltage of signal POR may be raised sufficiently soon and maintained at the raised voltage for a sufficient duration to enable reliable transfer of stored values from non-volatile memory elements NV₁ and NV₂ to initialize a memory state of bitcell circuit 1312 for operation as a volatile memory bitcell circuit.

According to an embodiment, bitcell circuit 1312 may be further modified to store a memory state in a single non-volatile memory element NV₁ as shown in bitcell circuit 1315 shown in the schematic diagram of FIG. 13D. Here, a value, symbol or condition expressed by a memory state maintained by PFETs P1 and P2, and NFETs N3 and N4 may be transferred to non-volatile memory element NV₁ (e.g., in a power down event) to be expressed or represented as a high impedance or insulative state, or a low impedance or conductive state. Likewise, a value, symbol or condition expressed by a memory state maintained by non-volatile memory element NV₁ (expressed or represented as a high impedance or insulative state, or a low impedance or conductive state) may be transferred to a volatile memory state maintained by PFETs P1 and P2, and NFETs N3 and N4 (e.g., in a power up event). The particular implementation of bitcell circuit 1315 does not include NFET N6 and NV₂ to enable implementation of a bitcell circuit volatile memory elements and non-volatile memory elements using fewer components.

In one implementation, bitcell circuit 1315 may transfer a non-volatile memory state maintained by NV₁ to a volatile memory state maintained by non-volatile memory elements PFETS P1 and P2, and NFETS N3 and N4 by initializing a state of a latch circuit formed by PFETs P1 and P2, and NFETs N3 and N4. As power is applied at voltage source 1306, NFET N5 may be turned on to close responsive to signal POR, connecting a terminal of NV₁ to node T (while disconnecting PFET P1 from node T). If NV₁ in a high impedance/resistance state (e.g., to store a first value, parameter, condition or symbol), node C may be placed at a higher voltage than node T, causing PFET P1 to be open and causing PFET P2 to be closed as part of the latch circuit including PFETs P1 and P2, and NFETs N3 and N4. Conversely, if NV₁ in a low impedance/resistance state (e.g., to store the second value, parameter, condition or symbol), node C may be placed at a lower voltage than node T, causing PFET P1 to be closed and causing PFET P2 to be open. Following connection of PFETs P1 and P2 to nodes T and C, respectively, signal POR may be lowered to open NFET N5 and disconnect non-volatile memory element NV₁, allowing bitcell circuit 1315 to operate as a volatile memory bitcell.

As pointed out above, it may be desirable is some scenarios of a computing platform to transfer values stored in a volatile memory state to a non-volatile memory state. Following transfer of values stored in a volatile memory state to a non-volatile memory state, for example, the computing platform may be powered down. Transferring the values stored in the volatile memory state to the non-volatile memory state and then back into the volatile memory state from the non-volatile memory state may allow the computing platform to quickly resume from a state occurring before the power down event (e.g., without having to load a memory state from an external non-volatile memory device which may be a much slower operation).

According to an embodiment, values stored in a volatile memory state at bitcell circuit 1300 may be transferred for storage in a non-volatile memory state at non-volatile memory elements NV₁ and NV₂. Similarly, values stored in a volatile memory state at bitcell circuit 1315 may be transferred for storage in a non-volatile memory state at non-volatile memory element NV₁. In one implementation, a value stored in a volatile memory state at bitcell circuit 1300 may be transferred for storage in a non-volatile memory state at non-volatile memory elements NV₁ and NV₂ (or just in non-volatile memory element NV₁) using a read-modify-write procedure. Likewise, a value stored in a volatile memory state at bitcell circuit 1315 may be transferred for storage in a non-volatile memory state at non-volatile memory element NV₁ similarly using a read-modify-write procedure.

In the particular implementation of bitcell circuit 1300, a volatile memory state may be detected in a read operation including, for example, connecting bitcell circuit 1300 to bitlines BL and BL′. For example, bitcell circuit 1300 may be accessed for a read operation by raising a voltage of wordline signal WL to close or enable NFETs N1 and N2. Bitlines BL and BL′ may be coupled to read circuit (not shown) capable of detecting the volatile memory state maintained at bitcell circuit 1300. The detected volatile memory state of bitcell circuit 1300 may be stored temporarily, and then written to non-volatile memory elements NV₁ and NV₂ in a subsequent write operation. For example, in the subsequent write operation, write driver circuits (not shown) coupled to bitlines BL and BL′ may apply programming signals based on the temporarily stored value. For example, bitcell circuit 1300 may be accessed again for a write operation by raising a voltage of wordline signal WL to close or enable NFETs N1 and N2 to connect bitlines BL and BL′ to bitcell circuit 1300. For example, while NFETs N1 and N2 are closed, write driver circuitry (not shown) may apply programming signals to non-volatile memory elements NV₁ and NV₂ to place non-volatile memory elements NV₁ and NV₂ in a memory state to represent values detected in the previous read operation. In a particular implementation in which non-volatile memory elements NV₁ and NV₂ comprise CES elements, such a programming signal may apply suitable voltages and current densities to non-volatile memory elements NV₁ and NV₂ (e.g., as described below in connection with FIGS. 14A and 14B) to place non-volatile memory elements NV₁ and NV₂ in complementary high impedance/insulative and low impedance/conductive states. A volatile memory state stored in bitcell circuit 1315 may be similarly transferred to a non-volatile memory state in non-volatile memory element NV₁.

In an alternative embodiment as shown in FIG. 13E, a volatile memory state maintained by PFETs P1 and P2, and NFETs N3 and N4 may be transferred to non-volatile memory elements NV₁ and NV₂ without accessing bitcell circuit 1320 through the enabling of the word line devise N1 and N2. In other words, a volatile memory state maintained in bitcell circuit 1320 by PFETs P1 and P2, and NFETs N3 and N4 may be transferred to non-volatile memory elements NV₁ and NV₂ without connecting bitlines BL and BL′ to bitcell circuit 1320 (e.g., by raising a voltage of wordline signal WL to close NFETs N1 and N2). Complementary write driver circuits 1326 and 1328 may then apply appropriate programming signals to non-volatile memory elements NV₁ and NV₂. As may be observed, transmission gate T2 of write driver circuit 1328 may be controlled based on a voltage at node T while transmission gate T1 of write driver circuit 1326 may be controlled based on a voltage at node C. In a particular implementation in which non-volatile memory elements NV₁ and NV₂ are formed as correlated electron switches, for example, a write driver circuits 1326 and 1328 may apply a programming signal comprising a voltage V_(set) at a current I_(set) to place a particular non-volatile memory element in a low impedance or conductive state. The particular elements indicated by 1322 and 1324 as part of the write drivers 1326 and 1328 may be coupled with T1 and T2 to create a combined voltage and current source and enable either a programming signal V_(set)/I_(set) (e.g., to place a non-volatile memory element in a low impedance or conductive state) or programming signal V_(reset)/I_(reset) (e.g., to place a non-volatile memory element in a high impedance or insulative state) through multiplexers 1302 and 1304. These elements may include a voltage source which supplies the appropriate voltage and current through T1 and T2 to enable non-volatile memory elements NV₁ and NV₂ to be written to appropriate states to maintain T and C when the power supply 1306 is removed and no power is applied to bitcell 1320.

As pointed out above, non-volatile memory bitcells described above may comprise bitcells including correlated electron switch (CES) elements incorporating a Correlated Electron Material (CEM). In this context, a CES element may exhibit an abrupt conductor/insulator transition arising from electron correlations rather than solid state structural phase changes (e.g., crystalline/amorphous in phase change memory (PCM) devices or filamentary formation and conduction in resistive RAM devices as discussed above). In one aspect, an abrupt conductor/insulator transition in a CES element may be responsive to a quantum mechanical phenomenon, in contrast to melting/solidification or filament formation. Such a quantum mechanical transition between conductive and insulative states in a CEM memory device may be understood in any one of several aspects.

In one aspect, a quantum mechanical transition of a CES element between an insulative state and a conductive state may be understood in terms of a Mott transition. In a Mott transition, a material may switch from an insulative state to conductive state if a Mott transition condition occurs. The criteria may be defined by the condition (n_(c))^(1/3) a=0.26, where n_(c) is a concentration of electrons and “a” is a Bohr radius. If a critical carrier concentration is achieved such that the Mott criteria is met, a Mott transition may occur and state may change from a high resistance/capacitance to a low resistance/capacitance.

In one aspect, a Mott transition may be controlled by a localization of electrons. As carriers are localized, a strong coulomb interaction between electrons splits the bands of the material creating an insulator. If electrons are no longer localized, a weak coulomb interaction may dominate band splitting, leaving behind a metal (conductive) band. This is sometimes explained as a “crowded elevator” phenomenon. While an elevator has only a few people in it, the people can move around easily, which is analogous to a conducting state. While the elevator reaches a certain concentration of people, on the other hand, passengers can no longer move, which is analogous to the insulative state. However, it should be understood that this classical explanation provided for illustrative purposes, like all classical explanations of quantum phenomenon, is only an incomplete analogy, and that claimed subject matter is not limited in this respect.

In particular implementations of aspects of this disclosure, a resistive switching integrated circuit memory may comprise: a resistive switching memory cell including a CES element; a write circuit for placing the resistive switching memory cell in a first resistive state or a second resistive state depending on signals provided to the memory cell, wherein the impedance of the CES element is higher in the second impedance state than in the first impedance state; and a read circuit for sensing the state of the memory cell and providing an electrical signal corresponding to the sensed state of the memory cell. In a particular implementation, a CES element may switch resistive states responsive to a Mott-transition in the majority of the volume of the CES element. In one aspect, a CES element may comprise a material selected from a group comprising aluminum, cadmium, chromium, cobalt, copper, gold, iron, manganese, mercury, molybdenum, nickel, palladium, rhenium, ruthenium, silver, tin, titanium, vanadium, and zinc (which may be linked to a cation such as oxygen or other types of ligands), or combinations thereof.

In a particular embodiment, a CES element may be formed as a “CEM random access memory (CeRAM)” device. In this context, a CeRAM device comprises a material that may transition between or among a plurality of predetermined detectable memory states based, at least in part, on a transition of at least a portion of the material between a conductive state and an insulative state utilizing the quantum mechanical Mott transition. In this context, a “memory state” means a detectable state of a memory device that is indicative of a value, symbol, parameter or condition, just to provide a few examples. In one particular implementation, as described below, a memory state of a memory device may be detected based, at least in part, on a signal detected on terminals of the memory device in a read operation. In another particular implementation, as described below, a memory device may be placed in a particular memory state to represent or store a particular value, symbol or parameter by application of one or more signals across terminals of the memory device in a write operation.

In a particular implementation, a CES element may comprise material sandwiched between conductive terminals. By applying a specific voltage and current between the terminals, the material may transition between the aforementioned conductive and insulative memory states. As discussed in the particular example implementations below, material of a CES element sandwiched between conductive terminals may be placed in an insulative or high impedance memory state by application of a first programming signal across the terminals having a voltage V_(reset) and current I_(reset), or placed in a conductive or low impedance memory state by application of a second programming signal across the terminals having a voltage V_(set) and current I_(set). In this context, it should be understood that terms such as “conductive or low impedance” memory state and “insulative or high impedance” memory state are relative terms and not specific to any particular quantity or value for impedance or conductance. For example, while a memory device is in a first memory state referred to as an insulative or high impedance memory state the memory device in one aspect is less conductive (or more insulative) than while the memory device in a second memory state referred to as a conductive or low impedance memory state. Furthermore, as discussed below with respect to a particular implementation, a CES element may be placed in any one of two or more different and distinguishable low impedance or conductive states.

In a particular implementation, CeRAM memory cells may comprise a metal/CEM/metal (M/CEM/M) stack formed on a semiconductor. Such an M/CEM/M stack may be formed on a diode, for example. In an example, implementation, such a diode may be selected from the group consisting of a junction diode and a Schottky diode. In this context, it should be understood that “metal” means a conductor, that is, any material that acts like a metal, including, for example, polysilicon or a doped semiconductor.

FIG. 14A shows a plot of current density versus voltage across terminals (not shown) for a CES element according to an embodiment. Based, at least in part, on a voltage applied to terminals of the CES element (e.g., in a write operation), the CES element may be placed in a conductive state or an insulative state. For example application of a voltage V_(set) and current density J_(set) may place the CES element in a conductive memory state and application of a voltage V_(reset) and a current density J_(reset) may place the CES element in an insulative memory state. Following placement of the CES element in an insulative state or conductive state, the particular state of the CES element may be detected by application of a voltage V_(read) (e.g., in a read operation) and detection of a current or current density at terminals of the CeRAM device.

According to an embodiment, the CES device of FIG. 14A may include any TMO, such as, for example, perovskites, Mott insulators, charge exchange insulators, and Anderson disorder insulators. In particular implementations, a CES device may be formed from switching materials such as nickel oxide, cobalt oxide, iron oxide, yttrium oxide, and perovskites such as Cr doped strontium titanate, lanthanum titanate, and the manganate family including praesydium calcium manganate, and praesydium lanthanum manganite, just to provide a few examples. In particular, oxides incorporating elements with incomplete d and f orbital shells may exhibit sufficient resistive switching properties for use in a CES device. In an embodiment, a CES device may be prepared without electroforming. Other implementations may employ other transition metal compounds without deviating from claimed subject matter. For example, {M(chxn)₂Br}Br₂ where M may comprise Pt, Pd, or Ni, and chxn comprises 1 R,2R-cyclohexanediamine, and other such metal complexes may be used without deviating from claimed subject matter.

In one aspect, the CES device of FIG. 14A may comprise materials that are TMO metal oxide variable resistance materials, though it should be understood that these are exemplary only, and are not intended to limit claimed subject matter. Particular implementations may employ other variable resistance materials as well. Nickel oxide, NiO, is disclosed as one particular TMO. NiO materials discussed herein may be doped with extrinsic ligands, which may stabilize variable resistance properties. In particular, NiO variable resistance materials disclosed herein may include a carbon containing ligand, which may be indicated by NiO(C_(x)). Here, one skilled in the art may determine a value of x for any specific carbon containing ligand and any specific combination of carbon containing ligand with NiO simply by balancing valences. In another particular example, NiO doped with extrinsic ligands may be expressed as NiO(L_(x)), where L_(x) is a ligand element or compound and x indicates a number of units of the ligand for one unit of NiO. One skilled in the art may determine a value of x for any specific ligand and any specific combination of ligand with NiO or any other transition metal simply by balancing valences.

If sufficient bias is applied (e.g., exceeding a band-splitting potential) and the aforementioned Mott condition is met (injected electron holes=the electrons in a switching region), the CES element may rapidly switch from a conductive state to an insulator state via the Mott transition. This may occur at point 1408 of the plot in FIG. 14A. At this point, electrons are no longer screened and become localized. This correlation may result in a strong electron-electron interaction potential which splits the bands to form an insulator. While the CES element is still in the insulative state, current may generated by transportation of electron holes. If enough bias is applied across terminals of the CES element, electrons may be injected into a metal-insulator-metal (MIM) diode over the potential barrier of the MIM device. If enough electrons have been injected and enough potential is applied across terminals to place the CES element in a particular low impedance or conductive state, an increase in electrons may screen electrons and remove a localization of electrons, which may collapse the band-splitting potential forming a metal.

According to an embodiment, current in a CES element may be controlled by an externally applied “compliance” condition determined based, at least in part, on an external current limited during a write operation to place the CES element in a conductive or low impedance state. This externally applied compliance current may also set a condition of a current density for a subsequent reset operation to place the CES element in a high impedance or insulative state. As shown in the particular implementation of FIG. 14A, a current density J_(comp) applied during a write operation at point 1416 to place the CES element in a conductive or low impedance state may determine a compliance condition for placing the CES device in a high impedance or insulative state in a subsequent write operation. As shown, the CES device may be subsequently placed in an insulative or high impedance state by application of a current density J_(reset)≥J_(comp) at a voltage V_(reset) at point 1408, where J_(comp) is externally applied.

The compliance therefore may set a number of electrons in a CES element which are to be “captured” by holes for the Mott transition. In other words, a current applied in a write operation to place a CES element in a conductive memory state may determine a number of holes to be injected to the CES element for subsequently transitioning the CES element to an insulative memory state.

As pointed out above, a reset condition may occur in response to a Mott transition at point 1408. As pointed out above, such a Mott transition may occur at condition in a CES element in which a concentration of electrons n equals a concentration of electron holes p. This condition may be modeled according to expression (1) as follows:

$\begin{matrix} {{{\lambda_{TF}n^{\frac{1}{3}}} = {C\text{\textasciitilde}0.26}}{n = \left( \frac{C}{\lambda_{TF}} \right)^{3}}} & (1) \end{matrix}$

where:

Δ_(TF) is a Thomas Fermi screening length; and

C is a constant.

According to an embodiment, a current or current density in a region 1404 of the plot shown in FIG. 14A may exist in response to injection of holes from a voltage signal applied across terminals of a CES element. Here, injection of holes may meet a Mott transition criterion for the conductive state to insulative state transition at current I_(MI) as a critical voltage V_(MI) is applied across terminals of CES element. This may be modeled according to expression (2) as follows:

$\begin{matrix} {{{I_{MI}\left( V_{MI} \right)} = {\frac{{dQ}\left( V_{MI} \right)}{dt} \approx \frac{Q\left( V_{MI} \right)}{t}}}{{Q\left( V_{MI} \right)} = {{qn}\left( V_{MI} \right)}}} & (2) \end{matrix}$

-   -   Where Q(V_(MI)) is the charged injected (hole or electron) and         is a function of an applied voltage.

Injection of electron holes to enable a Mott transition may occur between bands and in response to critical voltage V_(MI). and critical current I_(MI). By equating electron concentration n with a charge concentration to bring about a Mott transition by holes injected by I_(MI) in expression (2) according to expression (1), a dependency of such a critical voltage V_(MI) on Thomas Fermi screening length Δ_(TF) may be modeled according to expression (3) as follows:

$\begin{matrix} {{{I_{MI}\left( V_{MI} \right)} = {\frac{Q\left( V_{MI} \right)}{t} = {\frac{{qn}\left( V_{MI} \right)}{t} = {\frac{q}{t}\left( \frac{C}{\lambda_{TF}} \right)^{3}}}}}{{J_{reset}\left( V_{MI} \right)} = {{J_{MI}\left( V_{MI} \right)} = {\frac{I_{MI}\left( V_{MI} \right)}{A_{CeRam}} = {\frac{q}{A_{CeRam}t}\left( \frac{C}{\lambda_{TF}\left( V_{MI} \right)} \right)^{3}}}}}} & (3) \end{matrix}$

Where:

-   -   A_(CeRam) is a cross-sectional area of a CES element; and     -   J_(reset)(V_(MI)) is a current density through the CES element         to be applied to the CES element at a critical voltage V_(MI) to         place the CES element in an insulative state.

According to an embodiment, a CES element may be placed in a conductive memory state (e.g., by transitioning from an insulative memory state) by injection of a sufficient number of electrons to satisfy a Mott transition criteria.

In transitioning a CES to a conductive memory state, as enough electrons have been injected and the potential across terminal of the CES device overcomes a critical switching potential (e.g., V_(set)), injected electrons begin to screen and unlocalize double-occupied electrons to reverse a disproportion reaction and close the bandgap. A current density J_(set)(V_(MI)) for transitioning the CES to the conductive memory state at a critical voltage V_(MI) enabling transition to the conductive memory state may be expressed according to expression (4) as follows:

$\begin{matrix} {{{I_{MI}\left( V_{MI} \right)} = {\frac{{dQ}\left( V_{MI} \right)}{dt} \approx \frac{Q\left( V_{MI} \right)}{t}}}{{Q\left( V_{MI} \right)} = {{qn}\left( V_{MI} \right)}}{{I_{MI}\left( V_{MI} \right)} = {\frac{Q\left( V_{MI} \right)}{t} = {\frac{{qn}\left( V_{MI} \right)}{t} = {\frac{q}{t}\left( \frac{C}{a_{B}} \right)^{3}}}}}{{J_{set}\left( V_{MI} \right)} = {{J_{injection}\left( V_{MI} \right)} = {{J_{MI}\left( V_{MI} \right)} = {\frac{I_{MI}\left( V_{MI} \right)}{A_{CeRam}} = {\frac{q}{A_{CeRam}t}\left( \frac{C}{a_{B}} \right)^{3}}}}}}} & (4) \end{matrix}$

where:

a_(B) is a Bohr radius.

According to an embodiment, a “read window” 1402 for detecting an impedance state of a CES element in a read operation may be set out as a difference between a portion 106 the plot of FIG. 14A while the CES element is in an insulative state and a portion 104 of the plot FIG. 14A while the CES element is in a conductive state at a read voltage V_(read). In a particular implementation, read window 1402 may be used to determine a Thomas Fermi screening length λ_(TF) of material making up the CES element. For example, at a voltage V_(reset), current densities J_(reset) and J_(set) may be related to according to expression (5) as follows:

$\begin{matrix} {{\lambda_{TF}\left( {@V_{reset}} \right)} = {a_{B}\left( \frac{J_{reset}}{J_{off}} \right)}^{\frac{1}{3}}} & (5) \end{matrix}$

In another embodiment, a “write window” 1410 for placing a CES element in an insulative or conductive memory state in a write operation may be set out as a reset, difference between V_(reset) (at J_(reset)) and V_(set) (at J_(set)). Establishing |V_(set)|>|V_(reset)| enables a switch between conductive and insulative state. V_(reset) may be approximately at a band splitting potential arising from correlation and V_(set) may be approximately twice the band splitting potential. In particular implementations, a size of write window 1410 may be determined based, at least in part, by materials and doping of the CES element.

The transition from high resistance/capacitance to low resistance/capacitance in a CES element may be represented by a singular impedance of the CES element. FIG. 14B depicts a schematic diagram of an equivalent circuit of an example variable impeder device (such as a CES element), such as variable impeder device 1424. As mentioned, variable impeder device 1424 may comprise characteristics of both variable resistance and variable capacitance. For example, an equivalent circuit for a variable impeder device may, in an embodiment, comprise a variable resistor, such as variable resistor 1426 in parallel with a variable capacitor, such as variable capacitor 1428. Of course, although a variable resistor 1426 and variable capacitor 1428 are depicted in FIG. 14B as comprising discrete components, a variable impeder device, such as variable impeder device 1424, may comprise a substantially homogenous CEM element, wherein the CEM element comprises characteristics of variable capacitance and variable resistance. Table 1 below depicts an example truth table for an example variable impeder device, such as variable impeder device 1400.

TABLE 1 Resistance Capacitance Impedance R_(high)(V_(applied)) C_(high)(V_(applied)) Z_(high)(V_(applied)) R_(low)(V_(applied)) C_(low)(V_(applied))~0 Z_(low)(V_(applied))

In the particular implementation of a CES element of FIG. 14A, the CES element may be placed in either of two different impedance states: a low impedance or conductive state responsive to a set operation and a high impedance or insulative state responsive to a reset operation.

According to different embodiments, structures forming volatile memory bitcells and non-volatile memory bitcells (according to different implementations discussed above), may be integrated in a single integrated circuit (IC) or across multiple ICs. One particular implementation integrates structures forming volatile memory bitcells and non-volatile memory bitcells across multiple ICs in a three-dimensional (3D) IC 1500 as shown in FIG. 15. For example, layers 1502 in 3D IC 1500 may comprise monolithic ICs coupled by metallic interconnects wherein one particular layer 1502 comprises one or more memory arrays comprising exclusively volatile memory bitcells while a different particular layer 1502 comprises one or more memory arrays comprising exclusively non-volatile memory bitcells. In other implementations, structures forming volatile memory bitcells and non-volatile memory bitcells may be formed in a single layer 1502 and integrated according to embodiments discussed above.

A write operation performed in connection with particular embodiments described herein as a particular process of placing a memory device such as a CES element in a particular memory state of a plurality of predetermined memory states by applying a “programming signal” to terminals of the memory device. Particular ones of the predetermined memory states may correspond to particular voltage levels to be applied to the memory device (e.g., V_(set) and V_(reset)). Similarly, particular ones of the predetermined memory states may correspond to particular current levels to be applied to the memory device (e.g., I_(set) and I_(reset)). Accordingly, in a particular embodiment, a programming signal to place a CES device in a particular memory state in a write operation may be controlled to have a particular voltage level and current level corresponding to the particular memory state.

As described in a particular implementation below, a voltage signal having a voltage level for a programming signal to place a memory device in a predetermined memory state may be selected at a signal selection circuit based, at least in part, on a data signal. Conducting elements connected to the signal selection circuit may selectively connect the voltage signal to or disconnect the voltage signal from the memory device at a current level corresponding to the predetermined memory state based, at least in part, on the data signal. In this context, a “conducting element” comprises a circuit element capable of permitting current to pass between two nodes. In a particular implementation, a conducting element may vary a current permitted to pass between nodes based, at least in part, on a particular condition. The particular implementations described below employ FETs as conducting elements to permit current to pass between source and drain terminals based, at least in part, on a voltage applied to a gate terminal. It should be understood, however, that other types of devices such as a bipolar transistor, diode, variable resistor, etc. may be used as a conducting element, and that claimed subject matter is not limited in this respect. In this context, a conducting element having first and second terminals may “connect” the first and second terminals by providing a conductive path between the first and second terminals having a very small or negligible impedance for a particular signal. In one particular example implementation, a conductive element may vary an impedance between the first and second terminals based, at least in part, on a signal provided to a third terminal of the conductive element (e.g., a based on a voltage or current applied to the third terminal). In one aspect, a conductive element may “close” to thereby connect first and second terminals in response to a signal provided on the third terminal. Likewise, a conductive element may “open” to thereby disconnect first and second terminals in response to a different signal provide on the third terminal. In one aspect, a conductive element in an open state may isolate a first portion of a circuit from a second portion of the circuit by removing or disrupting a conductive path between the first and second portions of the circuit. In another aspect, a conducting element may vary an impedance between first and second terminals between opened and closed state based on a signal provided to a third terminal.

Particular embodiments described herein are directed to a bitcell circuit comprising: one or more volatile memory elements; one or more non-volatile memory elements, wherein a first volatile memory element of the one of the one or more volatile memory elements and at least a first non-volatile memory element of the one of the one or more non-volatile memory elements are accessible via a first bitline responsive to a signal on a wordline. In a particular implementation, the bitcell circuit further comprises a first conducting element to couple the first non-volatile memory element to a first node and second conducting element to couple a second non-volatile memory element to a second node to form a non-volatile memory cell that is responsive to a signal on a wordline to connect the non-volatile memory cell to the first bitline, and a third conducting element to couple the first volatile memory element to the first node and a fourth conducing element to couple a second volatile memory element to the second node to form a volatile memory cell that is responsive to a signal on said wordline to connect the volatile memory cell to said first bitline. In another particular implementation, the bitcell circuit further comprises at least a first multiplexer circuit to couple the first bitline to either a first terminal of the first volatile memory element or the first terminal of the first non-volatile memory element. In another particular implementation, the bitcell circuit further comprises a conducting element to selectively couple a terminal of the first non-volatile memory element to the first bitline to configure the bitcell circuit to operate as a non-volatile memory bitcell or decouple the terminal of the first non-volatile memory element from the first bitline to configure the bitcell circuit to operate as a volatile memory bitcell. In another particular implementation, the bitcell circuit further comprises a circuit to transfer a value stored in the first non-volatile memory element to the first volatile memory element responsive to power applied to the first volatile memory element. For example, the circuit to transfer the value stored in the first non-volatile memory element to the first volatile memory element is further configured to transfer the value stored in the first non-volatile memory element to the first volatile memory while the bitcell circuit is disconnected from the first bitline. In another example, the bitcell circuit is further configured to selectively decouple at least one terminal of the first non-volatile memory element from the bitline to configure the bitcell circuit to operate as a volatile memory bitcell. In another particular implementation, the bitcell circuit further comprises a write driver circuit to apply a programming signal to the first non-volatile memory element to affect a non-volatile memory state of the first non-volatile memory element based on a current volatile memory state of the first volatile memory element while the bitcell circuit is disconnected from the first bitline.

Particular embodiments described herein are directed to a method comprising: accessing a first non-volatile memory element in a bitcell circuit, the bitcell circuit comprising at least the first non-volatile memory element and a first volatile memory element accessible via a first bitline; and transferring a first value stored in the first non-volatile memory element to the first volatile memory element while the bitcell circuit is disconnected from the first bitline. In a particular implementation, transferring the value stored in the first non-volatile memory element to the first volatile memory element further comprises coupling a terminal of a PFET to a gate terminal of a first NFET. In another particular implementation, the method further comprises disconnecting the first non-volatile memory element from the gate terminal of the first NFET following transfer of the value stored in the first non-volatile memory element to the first volatile memory element. In another particular implementation, the method further comprises applying a programming signal to the non-volatile memory element to transfer a second value stored in the volatile memory element to the non-volatile memory element while the first bitline is disconnected from the non-volatile memory element. In another particular implementation, the method further comprises: coupling the volatile memory element to the bitline to detect a second value stored in the volatile memory element; and coupling the non-volatile memory element to the bitline to apply a programming signal to the non-volatile memory element to store the detected second value in the non-volatile memory element. In another particular implementation, the non-volatile memory element comprises a correlated electron switch (CES) device. In another particular implementation, transferring the value stored in the first non-volatile memory element to the first volatile memory element further comprises: connecting a terminal of the first non-volatile memory element to a node to place the node at a voltage that is based, at least in part, on the value stored in the first non-volatile memory element; and disconnecting the terminal of the first non-volatile memory element from the node. In another particular implementation, the bitcell circuit further comprises a second non-volatile memory element and a second volatile memory element accessible via a second bitline, and the method further comprises: transferring a value stored in the second non-volatile memory element to the second volatile memory element while the bitcell circuit is disconnected from the second bitline. For example, the method may further comprise applying power to the first and second non-volatile memory elements to place a first node coupled to a terminal of the first non-volatile memory element at a first voltage and place second node connected to a terminal of the second non-volatile memory element at a second voltage, wherein the first voltage is indicative of the value stored in the first non-volatile memory element and the second voltage is indicative of the value stored in the second non-volatile memory element. Also, the method may further comprise: applying the first voltage to a gate terminal of a first NFET; and applying the second voltage to a gate terminal of a second NFET, wherein transferring the value stored in the first non-volatile memory element to the first volatile memory element and transferring the value stored in the second non-volatile memory element to the second volatile memory element comprises placing a latch circuit in a first state or a second state responsive to application of the first voltage to the gate of the first NFET and application of the second voltage to the gate of the second NFET. In another particular implementation, the method further comprises: disconnecting the terminal of the first non-volatile memory element from the first node; and disconnecting the terminal of the second non-volatile memory element from the second node. In another particular implementation, the method further comprises operating the bitcell circuit as a volatile memory bitcell circuit following disconnecting the terminal of the first non-volatile memory element from the first node and disconnecting the terminal of the second non-volatile memory element from the second node. In another particular implementation, the method further comprises: connecting the first volatile memory element to the first bitline to read a value stored in the first volatile memory element; and connecting the first non-volatile memory element to the first bitline to write the read value stored in the first volatile memory element to the first non-volatile memory element. In another particular implementation, the method further comprises: writing a value stored in the first volatile memory element to the first non-volatile element while the bitcell circuit is disconnected from the first bitline. For example, writing the value stored in the first volatile memory element to the first non-volatile element while the bitcell circuit is disconnected from the first bitline may further comprise: generating a programming signal based, at least in part, on the value stored in the first volatile memory element; and applying the programming signal to terminals of the first non-volatile memory element.

Particular embodiments described herein are directed to a memory circuit comprising: one or more volatile memory elements enabling the bitcell circuit to operate in a volatile bitcell mode, the one or more volatile memory elements being accessible by at least a first bitline; and a circuit to selectively couple one or more non-volatile memory elements between the at least a first bitline and a voltage source to transition operation of the bitcell circuit from the volatile bitcell mode to a non-volatile bitcell mode. In one particular implementation, the bitcell circuit may further comprise a sense amplifier to detect a value stored in at least one of the one or more volatile memory elements while the at least one of the one or more volatile memory elements is connected to the bitline; and a write driver circuit to apply a programming signal to at least one of the one or more non-volatile memory elements to store the detected value in the at least one of the one or more non-volatile memory elements while the bitline is connected to the at least one of the one or more non-volatile memory elements. In a particular implementation, the memory circuit further comprises a circuit to transfer one or more values stored in the one or more volatile memory elements to the one or more non-volatile memory elements responsive to the transition of operation of the bitcell from the volatile bitcell mode to the non-volatile bitcell mode while the at least the first bitline is disconnected from the bitcell circuit.

References throughout this specification to one implementation, an implementation, one embodiment, an embodiment and/or the like means that a particular feature, structure, and/or characteristic described in connection with a particular implementation and/or embodiment is included in at least one implementation and/or embodiment of claimed subject matter. Thus, appearances of such phrases, for example, in various places throughout this specification are not necessarily intended to refer to the same implementation or to any one particular implementation described. Furthermore, it is to be understood that particular features, structures, and/or characteristics described are capable of being combined in various ways in one or more implementations and, therefore, are within intended claim scope, for example. In general, of course, these and other issues vary with context. Therefore, particular context of description and/or usage provides helpful guidance regarding inferences to be drawn.

While there has been illustrated and described what are presently considered to be example features, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from claimed subject matter. Additionally, many modifications may be made to adapt a particular situation to the teachings of claimed subject matter without departing from the central concept described herein. Therefore, it is intended that claimed subject matter not be limited to the particular examples disclosed, but that such claimed subject matter may also include all aspects falling within the scope of the appended claims, and equivalents thereof. 

What is claimed is:
 1. A bitcell circuit comprising: one or more volatile memory elements; one or more non-volatile memory elements, wherein a first volatile memory element of the one or more volatile memory elements and at least a first non-volatile memory element of the one or more non-volatile memory elements are accessible via a first bitline responsive to a signal on a wordline.
 2. The bitcell circuit of claim 1, and further comprising a first conducting element to couple the first non-volatile memory element to a first node and second conducting element to couple a second non-volatile memory element to a second node to form a non-volatile memory cell that is responsive to a signal on a wordline to connect the non-volatile memory cell to the first bitline, and a third conducting element to couple the first volatile memory element to the first node and a fourth conducing element to couple a second volatile memory element to the second node to form a volatile memory cell that is responsive to a signal on said wordline to connect the volatile memory cell to said first bitline.
 3. The bitcell circuit of claim 1, and further comprising at least a first multiplexer circuit to couple the first bitline to either a first terminal of the first volatile memory element or the first terminal of the first non-volatile memory element.
 4. The bitcell circuit of claim 1, and further comprise a conducting element to selectively couple a terminal of the first non-volatile memory element to the first bitline to configure the bitcell circuit to operate as a non-volatile memory bitcell or decouple the terminal of the first non-volatile memory element from the first bitline to configure the bitcell circuit to operate as a volatile memory bitcell.
 5. The bitcell circuit of claim 1, and further comprising a circuit to transfer a value stored in the first non-volatile memory element to the first volatile memory element responsive to power applied to the first volatile memory element.
 6. The bitcell circuit of claim 5, wherein the circuit to transfer the value stored in the first non-volatile memory element to the first volatile memory element is further configured to transfer the value stored in the first non-volatile memory element to the first volatile memory while the bitcell circuit is disconnected from the first bitline.
 7. The bitcell circuit of claim 6, wherein the bitcell circuit is further configured to selectively decouple at least one terminal of the first non-volatile memory element from the bitline to configure the bitcell circuit to operate as a volatile memory bitcell.
 8. The bitcell circuit of claim 1, and further comprising a write driver circuit to apply a programming signal to the first non-volatile memory element to affect a non-volatile memory state of the first non-volatile memory element based on a current volatile memory state of the first volatile memory element while the bitcell circuit is disconnected from the first bitline.
 9. The bitcell circuit of claim 1, wherein the one or more non-volatile memory elements comprise one or more correlated electron switch (CES) elements.
 10. A method comprising: accessing a first non-volatile memory element in a bitcell circuit, the bitcell circuit comprising at least the first non-volatile memory element and a first volatile memory element accessible via a first bitline; and transferring a first value stored in the first non-volatile memory element to the first volatile memory element while the bitcell circuit is disconnected from the first bitline.
 11. The method of claim 10, wherein the non-volatile memory element comprises a correlated electron switch (CES) device.
 12. The method of claim 10, wherein transferring the value stored in the first non-volatile memory element to the first volatile memory element further comprises: connecting a terminal of the first non-volatile memory element to a node to place the node at a voltage that is based, at least in part, on the value stored in the first non-volatile memory element; and disconnecting the terminal of the first non-volatile memory element from the node.
 13. The method of claim 10, wherein the bitcell circuit further comprises a second non-volatile memory element and a second volatile memory element accessible via a second bitline, and further comprising: transferring a value stored in the second non-volatile memory element to the second volatile memory element while the bitcell circuit is disconnected from the second bitline.
 14. The method of claim 13, and further comprising: applying power to the first and second non-volatile memory elements to place a first node coupled to a terminal of the first non-volatile memory element at a first voltage and place second node connected to a terminal of the second non-volatile memory element at a second voltage, wherein the first voltage is indicative of the value stored in the first non-volatile memory element and the second voltage is indicative of the value stored in the second non-volatile memory element.
 15. The method of claim 14, and further comprising: applying the first voltage to a gate terminal of a first NFET; and applying the second voltage to a gate terminal of a second NFET, wherein transferring the value stored in the first non-volatile memory element to the first volatile memory element and transferring the value stored in the second non-volatile memory element to the second volatile memory element comprises placing a latch circuit in a first state or a second state responsive to application of the first voltage to the gate of the first NFET and application of the second voltage to the gate of the second NFET.
 16. The method of claim 14, and further comprising: disconnecting the terminal of the first non-volatile memory element from the first node; and disconnecting the terminal of the second non-volatile memory element from the second node.
 17. The method of claim 14, and further comprising operating the bitcell circuit as a volatile memory bitcell circuit following disconnecting the terminal of the first non-volatile memory element from the first node and disconnecting the terminal of the second non-volatile memory element from the second node.
 18. The method of claim 10, and further comprising: connecting the first volatile memory element to the first bitline to read a value stored in the first volatile memory element; and connecting the first non-volatile memory element to the first bitline to write the read value stored in the first volatile memory element to the first non-volatile memory element.
 19. The method of claim 10, and further comprising: writing a value stored in the first volatile memory element to the first non-volatile element while the bitcell circuit is disconnected from the first bitline.
 20. The method of claim 19, wherein writing the value stored in the first volatile memory element to the first non-volatile element while the bitcell circuit is disconnected from the first bitline further comprises: generating a programming signal based, at least in part, on the value stored in the first volatile memory element; and applying the programming signal to terminals of the first non-volatile memory element.
 21. A memory circuit comprising: one or more volatile memory elements enabling the bitcell circuit to operate in a volatile bitcell mode, the one or more volatile memory elements being accessible by at least a first bitline; and a circuit to selectively couple one or more non-volatile memory elements between the at least a first bitline and a voltage source to transition operation of the bitcell circuit from the volatile bitcell mode to a non-volatile bitcell mode.
 22. The memory circuit of claim 21, and further comprising: a sense amplifier to detect a value stored in at least one of the one or more volatile memory elements while the at least one of the one or more volatile memory elements is connected to the bitline; and a write driver circuit to apply a programming signal to at least one of the one or more non-volatile memory elements to store the detected value in the at least one of the one or more non-volatile memory elements while the bitline is connected to the at least one of the one or more non-volatile memory elements.
 23. The memory circuit of claim 21, and further comprising a circuit to transfer one or more values stored in the one or more volatile memory elements to the one or more non-volatile memory elements responsive to the transition of operation of the bitcell from the volatile bitcell mode to the non-volatile bitcell mode while the at least the first bitline is disconnected from the bitcell circuit. 